1. Field of the Invention
This invention relates generally to a system and method for controlling an automatic transmission in a vehicle and, more particularly, to a system and method for preventing a change in torque converter slip and/or transmission gear shift position in response to a sudden increase in the acceleration of the transmission or vehicle speed as a result of wheel slip or any event that may cause an abrupt change in driveline speed.
2. Discussion of the Related Art
Internal combustion engine vehicles that employ automatic transmissions typically include a torque converter positioned between the engine and the transmission of the vehicle. A torque converter is a fluid coupling device typically including an impeller coupled to an output shaft of the engine and a turbine coupled to the input shaft of the transmission. The torque converter uses hydraulic fluid to transfer rotational energy from the impeller to the turbine. Thus, the torque converter can disengage the engine crank shaft from the transmission input shaft during vehicle idling conditions to enable the vehicle to stop and/or to shift gears.
The rotational speed of the impeller relative to the turbine in the torque converter is typically different so that there is a converter slip therebetween. Because large slips between the engine output and the transmission input significantly affect the fuel economy of the vehicle, vehicles employ a torque converter clutch (TCC) for controlling or reducing the slip between the engine and the transmission. The TCC can also mechanically lock the impeller at the output of the engine to the turbine at the input of the transmission so that the engine and transmission rotate at the same speed. Locking the impeller to the turbine is generally only used in limited circumstances because of various undesirable implications.
Thus, a TCC generally has three modes. A fully locked mode as just described, a fully released mode and a controlled slip mode. When the TCC is fully released, the slip between the impeller and the turbine of the torque converter is only controlled by the hydraulic fluid therebetween. In the slip mode, the TCC is controlled by the pressure of hydraulic fluid in the torque converter so that the slip between the torque converter impeller and the turbine can be set so that it does not exceed a predetermined slip.
A conventional gearshift schedule for automatic transmissions is implemented in the form of shift maps. A shift map is a group of table look-up functions that define gearshift points based on vehicle speed and throttle opening. Consequently, each gearshift point defined using conventional gearshift scheduling is a function of vehicle speed and throttle opening, and does not compensate for varying vehicle conditions, such as varying vehicle loads, or road conditions, such as road grade and curvature. However, it is desirable to dynamically generate gearshift points that use a combination of inputs that compensate for varying vehicle and road conditions to achieve optimal fuel economy, performance and drivability.
As a vehicle travels along the road, it is not uncommon that the conditions of the road can affect the speed of the driveline and transmission of the vehicle. For example, when one or more of the vehicle tires lose contact with the road surface as a result of road bumps or the like, vehicle slip may occur which causes the driveline to spin much faster in a short period of time. When the change in speed between the transmission and the engine is detected by various speed sensors, typically the transmission controller will cause the torque converter clutch to increase the converter slip between the engine and the transmission, possibly causing TCC lock-up. The abrupt change in the speed also can trigger a gear shift. Therefore, it would be desirable to detect such a condition so that the transmission controller can prevent the TCC from locking up or changing gear, which is generally undesirable.